This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to improving image slice selection by correcting RF amplifier distortion.
In MRI, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example at 90.degree. or 180.degree.. After excitation the nuclei gradually return to with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, ant he FID frequency, is defined by the Larmor relationship which states that the angular frequency .omega..sub.0, of the precession of the nuclei is the product of the magnetic field B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .multidot..gamma..
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field B.sub.0, which defines Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
In the magnetic resonance imaging system, a radio-frequency (RF) power amplifier is employed to boost an RF pulse to sufficient strength to excite the nuclear spins in a subject. In the design of the amplifier, precision is often compromised in favor of power output and efficiency. Consequently, a fair amount of distortion can be introduced into a selective-excitation pulse, and this in turn degrades the definition of a slice profile by widening transition widths, reducing in slice flatness, and increasing out-of-slice signal. Poor profile of definition adversely effects imaging such as multi-slice, MR angiography, and two-dimensional excitation imaging.